The present invention relates to a light receiving and emitting compound element which is suitable for modifying focus errors and tracking errors of an optical pick-up device to be used for recording and reproducing an optical disk such as a CD-ROM, a CD-R, a DVD-ROM or an MD and correcting an image forming position to automatically form an image of a microdisplay provided in front of eyes on a human retina, and to an optical pick-up device using the light receiving and emitting compound element. More specifically, the present invention relates to a photodetector capable of accurately detecting reflected light corresponding to the situation of reflection on an object without precisely adjusting the position of an optical system to align a light emitting portion with a light receiving portion and of detecting a shift in a direction of a focal point and a shift in an xy plane (in a tracking direction), and to an optical pick-up device using the photodetector.
A pit for recording information of an optical disk acting as an information recording medium has a size of approximately 0.9 xcexcm, and a train (track) of the pit is arranged with a small pitch of approximately 1.6 xcexcm. In an actual optical pick-up device, therefore, a pit should be detected while correcting a shift in a focus or the like which is caused by a displacement in a direction perpendicular to a direction of the track, a rotation slippage of the optical disk or the like. As a method for correcting the shift in the focus, there have conventionally been used an astigmatism method comprising the steps of separating a received signal light reflected by an optical disk from a transmitted signal light through a half lens or a beam splitter, forming an astigmatism through a cylindrical lens or the like and receiving the light by means of a 4-split sensor to detect a shift in a focus, a Foucault method for detecting a shift in a focus depending on whether or not a light reflected by an optical disk forms a focal point on an apex angle of a prism by using the prism, for example, and the like.
As an example of an optical pick-up device using the astigmatism method, for example, as shown in FIG. 15, a light transmitted from a light source such as a laser diode (which will be hereinafter referred to as an LD) 1 is diffracted through a diffraction grating 6 to generate a beam for a tracking servo (a 3-beam method), the beam is reflected by a half mirror 7 and is collected on a pit of an optical disk 10 through a lens system 3 including a collimator 3a, an objective lens 3b and the like, the reflected light is received by a light receiving element 2 such as a photodiode through the objective lens 3b, the collimator 3a, the half mirror 7 and a concave lens 8, and a size of the pit of the optical disk 10 is detected while carrying out a focus servo and a tracking servo so that information is read out.
In this example, a half mirror plane is formed as the half mirror 7 on a surface of a thick transparent substrate without using a cylindrical lens. Consequently, 50% of the light transmitted from the LD1 is reflected by the surface and is transmitted to the optical disk 10 without a distortion, and half of the light reflected by the optical disk 10 and returned is refracted through the thick half mirror 7 and is then transmitted to the light receiving element 2. In this case, the half mirror 7 has an inclination in a constant direction in order to reflect the light transmitted from the LD1 toward the optical disk 10 side. All convergent beams reflected and returned which enter an x-axis of FIG. 15 have equal angles of incidence with respect to the inclined surface of the half mirror 7, while convergent beams entering a y-axis have different angles of incidence with respect to the inclined surface in a y-axis direction. For this reason, while a position of a convergent point is changed, all the beams in an x direction which enter the x-axis are refracted in the same manner and are simply moved in parallel so that a distortion is not generated. Consequently, an astigmatism is generated. By utilizing the astigmatism, the focus servo is carried out.
The example using a hologram applying the Foucault method, moreover, as shown in FIG. 16, a laser beam transmitted from the LD1 is diffracted through the diffraction grating 6 to generate a beam for a tracking servo (the 3-beam method) and is then collected on a pit of the optical disk 10 through a hologram (diffracting element) 9 provided on the top surface of the diffraction grating 6 and the lens system 3 including the collimator 3a and the objective lens 3b, the reflected light is transmitted through the objective lens 3b, the collimator 3a and the hologram 9, and a primary diffraction signal diffracted through the hologram 9 is received by the light receiving element 2 so that information is read out in the same manner described above. The hologram 9 has the functions of a plane beam splitter and a Foucault prism in the conventional optical pick-up device and uses the principle of the Foucault method.
These methods have a problem in that the light emitting element 1 and the light receiving element 2 are provided in different positions and are therefore aligned with much difficulty. More specifically, an area of the light receiving element 2 should be very reduced in order to recognize a shift in a focal point or the like. In order to receive the reflected light through the very small light receiving element 2, the light receiving element 2 should be provided accurately in the convergent position of the reflected light. As a method for solving such a problem, there has been known a structure shown in FIG. 17.
A structure shown in FIG. 17(a) is referred to as a so-called L-SCOOP (Self Coupled Optical Pickup) method utilizing a change in an amount of detection through a light receiving element 61 for a monitor provided behind the LD1 because a light irradiated from the LD1 and reflected by the optical disk 10 and then returned is exactly returned to the LD1 and an oscillation state of the LD1 is changed by the reflected light thus returned. A structure shown in FIG. 17(b) is referred to as a so-called E-SCOOP method in which a change in the oscillation state of the LD1 through the reflected light is directly detected depending on a change in a driving current or terminal voltage of the LD in the same manner.
In the case in which a light beam transmitted from a light source is irradiated on an object such as an optical disk and the reflected light is received to detect information or the like as in the above-mentioned optical pick-up device, light paths for reciprocation should be separated by using a beam splitter, a hologram or the like and a light emitting element and a light receiving element should be assembled by separate parts. For this reason, it is necessary to carefully adjust mutual optical axes of the light emitting element and the light receiving element and a distance therebetween, thereby carrying out assembly. However, the work is very complicated and only the skilled can carry out the assembly.
In the optical pick-up device using the astigmatism method, an expensive part such as a hologram is not required and it is sufficient that a half mirror plane is formed on the surface of the transparent substrate. Although parts are inexpensive, this structure also requires a half mirror and uses the 3-beam method to require a diffraction grating. All these parts are discrete and the number of the parts is increased, resulting in an increase in a cost. In addition, the relationship between the LD and the light receiving element which are aligned carefully should be prevented from being changed due to a temperature or the like. Therefore, a case or the like should be manufactured by an expensive material such as engineering plastics which is deformed by heat with difficulty. Consequently, the cost of the parts is increased. For this reason, there is a problem in that a manufacturing cost is increased.
In the SCOOP method, moreover, the alignment can be carried out very easily. However, the amount of a change caused by a shift is very small and reading reliability is maintained with difficulty. Therefore, the SCOOP method has not been put into practical use.
In order to solve the problems, it is an object of the present invention to provide a light receiving and emitting compound element capable of detecting a change in a reflected light due to a shift with a high sensitivity without adjusting the positional relationship between a light emitting portion and a light receiving portion, thereby surely detecting a focal position of an object and the like.
It is another object of the present invention to provide an optical pick-up device capable of being assembled easily and of detecting and correcting a shift of a focus or tracking with a high sensitivity by using the light receiving and emitting compound element.
It is yet another object of the present invention to provide an optical pick-up device in which a light emitting portion and a light receiving portion are formed in almost the same places, a beam irradiated from the light emitting portion to an optical disk rarely causes an astigmatism and a light reflected from the optical disk and returned to the light receiving portion causes the astigmatism, and a focus servo can be carried out by an astigmatism method.
It is a further object of the present invention to provide an optical pick-up device capable of being assembled very easily without using an expensive part such as a diffraction grating also in the case in which a 3-beam method is used.
In order to solve the above-mentioned problems, the present inventor has made investigations vigorously. As a result, the following has been found. More specifically, a reflected light passing through an objective lens has such a property that a diameter of a beam (beam waist) which becomes the thinnest during collection is determined to be constant depending on a numerical aperture NA of the lens by the influence of diffraction of the lens, and the light emitting portion and the light receiving portion are formed in the beam diameter by utilizing the property so that only an intensity of the reflected light can be detected separately from the light emitting portion without separating the reflected light from a main beam.
A light receiving and emitting compound element according to the present invention comprises; a light emitting portion to irradiate a light toward an object through an objective lens for collecting the light; and a light receiving portion to detect the light collected by the objective lens and reflected by the object and to check a positional relationship between the objective lens and the object based on an intensity of the detected light, wherein the light emitting portion and the light receiving portion are provided within a range of 1.22 xcex/NA, in which a numerical aperture of the objective lens is represented by NA and a wavelength of the light of the light emitting portion is represented by xcex, and an intensity of the reflected light can be detected by the light receiving portion.
With such a structure, the light emitting portion and the light receiving portion are formed in the beam waist of the reflected light. Therefore, the light receiving portion is present in a beam to be received if focal points are coincident with the surface of the object. And a beam diameter of the reflected light is large if the surface of the object is not focused, consequently, the amount of the received light is reduced. If the focal points are coincident with the surface of the object, all the reflected lights are detected so that the amount of the received light is increased. Thus, an intensity of the reflected light can be detected accurately. As a result, it is possible to detect that the object is set on a focusing point or a non-focusing point. Correspondingly, the position of the objective lens can be adjusted.
The present invention provides another form of a light receiving and emitting compound element comprises: a light emitting portion and a light receiving portion for receiving a light transmitted from the light emitting portion which is reflected by an object, thereby detecting information of the object, wherein the light receiving portion is multi-split and the light emitting portion and the light receiving portion are formed on the same substrate such that the light emitting portion is positioned on a central part of the light receiving portion.
With such a structure, for example, by generating an astigmatism in only the light reflected by the object and returned to the light receiving portion, even if the light transmitted from the light emitting portion and the light returned to the light receiving portion are not separated by the optical system, the focal position of the optical system can be automatically adjusted such that the beam of the light emitting portion forms an image on the object by the astigmatism method.
For example, the light emitting portion and the light receiving portion are formed on a single semiconductor substrate to be electrically isolated from each other.
The light receiving portion may be formed by a semiconductor substrate and the light emitting portion may be an organic EL element provided on the semiconductor substrate, and the light emitting portion, or the light receiving portion may be formed by a surface emitting laser and a photodiode which are formed on the same semiconductor substrate.
A second light receiving portion is further provided on an outside of the range of 1.22 xcex/NA to be electrically isolated. Therefore, also in the case in which a shift is great in the xy plane so that the reflected light is not detected in the light receiving portion at all, for example, the reflected light can be detected by the second light receiving portion so that it can be known that a shift is great.
An optical pick-up device according to the present invention comprises: a light receiving and emitting compound element having a light emitting portion to irradiate a light toward an optical disk and a light receiving portion to detect the light reflected by the optical disk; an objective lens for collecting the light of the light emitting portion; a tracking servo mechanism and a focus servo mechanism which are driven through an output of the light receiving and emitting compound element wherein the light emitting portion and the light receiving portion are provided within a range of 1.22 xcex/NA, in which a numerical aperture of the objective lens is represented by NA and a wavelength of the light of the light emitting portion is represented by xcex, and a positional relationship between the objective lens and the optical lens based on an intensity of the reflected light can be detected by the light receiving portion.
The present invention provides a further form of an optical pick-up device comprising a light emitting portion, a lens system for collecting a light transmitted from the light emitting portion on an optical disk, and a light receiving portion for receiving the light reflected by the optical disk, thereby detecting information of the optical disk, wherein the light receiving portion is formed on the same substrate as the light emitting portion to be positioned around the light emitting portion, the light receiving portion is multi-split, and a focal position on the optical disk of the light transmitted from the light emitting portion can be detected by an astigmatism method. The multi-split implies that the split can be carried out to compare the amount of the received light in the x-axis direction with the amount of the received light in the y-axis direction, for example, that four-split is carried out.
With such a structure, the light emitting portion and the light receiving portion are formed on the same substrate by using the astigmatism method without a beam splitter such as a half mirror. Therefore, if the light emitting portion and the lens system are aligned, the receiving portion can be aligned automatically and assembly can be carried out very easily. Furthermore, the light emitting portion and the light receiving portion are formed integrally. Therefore, there is no possibility that a relative position might be moved by heat or the like. In addition, a case has a simple structure and the cost of parts can be reduced considerably.
In a specific structure for carrying out the focus servo by the astigmatism method, a first optical element for generating an astigmatism on the light reflected by the optical disk and returned is provided on only a surface of the light receiving portion. The optical element for generating an astigmatism implies an element such as a cylindrical lens and an optical element of which focal position is shifted on the x axis and y axis such as a parallel transparent substrate provided to have a constant inclination with respect to a beam axis.
In another specific structure for carrying out the focus servo by the astigmatism method, a second optical element for generating an astigmatism is provided on only a front surface of the light emitting portion and a third optical element for offsetting the astigmatism generated by the second optical element is provided between the light emitting portion and the optical disk to cover a portion of the light returned to the light receiving portion or the light emitting portion is formed to cause an optical beam emitted from the light emitting portion to have an astigmatism and a third optical element for offsetting the astigmatism is provided between the light emitting portion and the optical disk to cover a portion of the light returned to the light receiving portion.
The third optical element for offsetting the astigmatism implies such an optical element as to generate an astigmatism at almost the same rate in a reverse direction to that of the second optical element. For example, it is possible to use a cylindrical lens having a concave portion for canceling the effects of the convex cylindrical lens, a transparent substrate inclined in such a direction as to cancel the lens effects of the second optical element and the like.
Furthermore, a fourth optical element for generating a small astigmatism may be inserted between the light emitting portion as well as the light receiving portion and the optical disk and may be formed to generate such an astigmatism that an optical beam transmitted from the light emitting portion is collected on the optical disk with a spot having no hindrance when a signal of the optical disk is to be detected. Thus, when the beam transmitted from the light emitting portion is to be collected on the optical disk, a small astigmatism is generated. If the minimum diameter of the beam spot based on the astigmatism is approximately 2 xcexcm or less, information can be detected without a hindrance. The light reflected by the optical disk and returned to the light receiving portion is transmitted through the fourth optical element for generating an astigmatism again. Therefore, the astigmatism becomes a double so that a focal shift can be detected by the astigmatism method.
Additional two sets of light emitting portions and light receiving portions which can detect a shift in a track of the optical disk are formed on the same substrate as that on which the multi-split light receiving portion is provided in addition to the light emitting portion for detecting a focal position and the multi-split light receiving portion. Thus, since the light emitting portion and the light receiving portion are simultaneously formed on the same substrate, a very accurate positional relationship can be obtained, an expensive part such as a diffraction grating is not employed and a beam can be used as a signal for a tracking servo.